1. Field of the Invention
The present invention relates to a circuit arrangement for controlling an illuminating device, in particular light-emitting diodes, in a control device of a motor vehicle, comprised of at least one illuminating device integrated in a switching circuit having a semiconductor component, and a microprocessor, in particular with an integrated non-volatile memory, for generating a pulse-width modulated signal, whereby the pulse-width modulated signal is connected to the control electrode of the semiconductor component.
2. Description of the Background Art
It is known to provide control elements in motor vehicles with an illumination. In order to adapt the illumination, or rather the backlighting, of the control elements and displays to ambient light effects, the illumination is adjustable. A distinction is thereby made between a day and night illumination on the one hand, and between a locator and function illumination on the other hand. Depending on the variation of interior design in the motor vehicle, it is also known to adapt, via a photo-sensitive sensor, these different types of lighting to the incident light striking the control element directly. There are different methods known for controlling the illuminating means.
Light-emitting diodes (LEDs) are typically used for as illuminating devices in control devices. The objective thereby is to achieve a harmonious, evenly illuminated layout of the control device. The luminosity of LEDs or lamps, however, tends to be very widely scattered so that the tolerances stipulated by the automobile industry can frequently not be achieved without further measures. From DE 196 02 891 A1, it is known to determine, after the manufacture of the control device, a correction factor depending on the brightness of the illuminating means, with which the control voltage or the control current of the illuminating means is adjusted.
In order for the described types of illumination to be even continuously adjustable and controllable, if need be, it is known from the cited publication to control the illuminating means via a semiconductor component, and in particular via a pulse-width modulated signal (PWM). With the assistance of a PWM signal present at the output of a microprocessor, a transistor, for example, is clock-controlled, which allows for the brightness of the light-emitting diode to be adjusted with very great precision. Due to the modulation of the pulse width, the brightness of the light of the illuminating means can thus be altered, whereby the duty cycle, that is, the ratio of impulse duration to cycle duration, represents a direct measure of the brightness of the illuminating means.
In order to be able to regulate the illumination of a combined locator and function symbol via a PWM signal of a control device, which can be a microcomputer, for example, in dependence from incident sunlight, it is known from DE 102 09 622 A1, which corresponds to U.S. Pat. No. 7,022,961, to detect the brightness in the ambient area of the control element via an optoelectric sensor. The influence of a non-linearity of the light-current characteristic line of the respective LED, which occurs at high currents, for example, self-heating of the LEDs, can hereby be adjusted by way of a characteristic line stored in a software of a control unit, whereby the PWM ratio is preferably adapted.
It is known from patent application DE 102 54 741 to influence a pulse-width modulated signal for the activation of LEDs in the duty cycle. For this purpose, the PWM signal generated in the microcomputer in a circuit arrangement having logical connectives is influenced such that only every other impulse of the PWM signal is transmitted to the illuminating means.